This invention relates generally to conveyors and, more particularly, to accumulation conveyor systems that achieve zero back pressure by selectively coupling articles to and decoupling them from a transport belt in individual accumulation zones along a conveying path.
When articles being conveyed on a moving conveyor start to back up, trailing articles push against leading articles. The result is a buildup of backline pressure, which is greatest on the lead articles. Too much backline pressure can crush or otherwise damage the articles and load the conveyor because of the dynamic friction between the moving conveyor and the stalled or slowly-moving articles.
In the corrugated industry, stacks of cardboard sheets are conveyed along a processing line. A common way to convey these stacks is with powered roller conveyors. In these conveyhors, parallel cylindrical rollers with axes of rotation transverse to the conveying direction are arranged to form a rolling conveyor bed. Drive belts are often used to contact the rollers to rotate them and propel the stacks along the roller bed. To eliminate backline pressure by preventing consecutive stacks from bumping into each other, the roller conveyor is divided into successive accumulation zones. The rollers in one zone are powered independently of those in another zone. In this way, when a downstream stack is stopped in one zone of the conveyor, the trailing upstream stack can be moved from zone to zone and stopped in the zone just upstream of the stopped downstream stack without contact. Various drive arrangements are used to achieve individual zonal control by selectively engaging the rollers in each zone with the drive belt.
In another version, a conveyor belt is flanked on each side by a roller conveyor bed. The stack of cardboard sheets rests atop both roller conveyor belts. Portions of the conveyor belt are raised and lowered into and out of contact with the bottom of the stacks. When raised into contact, the conveyor belt transports the stack along; when the belt is lowered out of contact, the stack rests in place on the two roller conveyor beds. Thus, each portion of the conveyor belt that can be raised and lowered defines an accumulation zone.
But these zero-back-pressure roller conveyors have shortcomings. The rollers have a tendency to freeze up or their mounting holes to wallow out over time, resulting in such performance deficiencies as increased friction against the conveyed stacks, a bumpy conveyor bed, and excessive noise. Roller conveyors also cause a stack of cardboard to form an xe2x80x9celephant footxe2x80x9d as it is conveyed. There are a couple of causes for the xe2x80x9celephant foot.xe2x80x9d As the stack traverses the spacing between consecutive rollers, the leading edge of the bottom-most sheets bumps into the upcoming roller. Each time this occurs, the sheets above tend to creep forward relative to the bottom sheets. Article creep is also caused by a wave effect. The weight of the stack on the bottom-most sheets makes them conform to the rollers. The closer a sheet is to the bottom of the stack, the more it deforms around the rollers into the inter-roller gaps and adopts a wavy shape. As the stack moves over the rollers, the wave dynamically propagates. upward into the stack, causing adjacent sheets at the bottom of the stack to creep. On a long conveying path over many rollers, the side profile of the stack resembles an xe2x80x9celephant footxe2x80x9d with the leading edge of the bottom-most sheet lagging the leading edge of the topmost sheets. If the xe2x80x9celephant footxe2x80x9d becomes too exaggerated, the stack becomes unstable, and sheets tip over, requiring manual intervention to rearrange the stack.
One way to achieve zero back pressure and minimize the xe2x80x9celephant footxe2x80x9d problem is to use a series of conveyor belts, or chains, arranged end to end with a small space between consecutive belts. Each belt, which forms an accumulation zone, is individually controlled by its own drive train and sprockets or pulleys. The flat conveying surfaces provided by the belts avoid the bumpiness of a roller conveyor, and the xe2x80x9celephant footxe2x80x9d problem is minimized. But such an arrangement is more complex and costly in that multiple sprockets, shafts, and drive motors are required to handle all of the zones, especially in a long conveyor system.
Thus, there is a need for a zero-back-pressure conveyor that eliminates the problems with transporting sheets of cardboard and other articles and that lacks the shortcomings of conventional conveyor systems.
These needs and others are satisfied by a zero-back pressure conveyor having features of the invention. In one embodiment, the conveyor includes a transport belt flanked on opposite sides by a first and a second series of inverted roller belt loops arranged in pairs. The transport belt forms an upper carryway for transporting articles in a transport direction between the two series of inverted roller belt loops, each of which forms a top supportway. The supportways of each pair of belt loops are generally coplanar and jointly support an article. The transport belt and each pair of inverted roller belt loops are relatively positionable between a first position in which the upper carryway of the transport belt is generally coplanar with the supportways of the pair of flanking belt loops and a second position in which the carryway is below the plane of the supportways. Thus, in the first position, the transport belt is positioned to contact conveyed articles to move them in the transport direction; and, in the second position, the transport belt is out of contact with the articles. In this way, zero-back-pressure accumulation zones are formed by each pair of inverted roller belt loops.
In one version of the zero-back-pressure conveyor, the position of the carryway relative to the supportways is controlled by a belt-positioning mechanism that raises and lowers the transport belt in each accumulation zone between a first position with the transport belt contacting the conveyed article and a second position with the transport belt out of contact with the article. In one simple embodiment, the belt-positioning mechanism includes an inflatable air tube in each accumulation zone supported in a pan. A slider bed supported on the air tube, in turn, supports the transport belt. A controller controls a supply of air to inflate and deflate the air tube and thereby raise and lower the slider bed and the transport belt into and out of contact with supported articles in each accumulation zone. Because the articles are jointly supported across the stationary supportways, the belt-positioning mechanism has to lift only the transport belt and not the weight of the articles.
One version of the inverted roller belt loop is constructed of a plurality of belt modules and hinge pins. Each module. includes a module body that extends from a first end to a second end in a roll direction and through the body""s thickness from a first side to an opposite second side. The first side has a generally flat surface. A roller is positioned on the second side to enable the module body to be rolled in the roll direction. Hinge elements along the first end are interleaved with hinge elements along the second end of an adjacent module and interconnected by hinge pins into an endless articulating belt loop. The modules are arranged with the first flat sides outward and the roller-topped sides inward to form the inverted roller belt loop. Articles on the flat supportway surface of the inverted roller belt loop are supported stably in each accumulation zone. By their low-friction contact with the roller bed, the rollers allow the conveyed articles to move with the inverted roller belt loop without slipping. The extended flat supportway is especially effective in eliminating the bumpy ride to which articles, such as stacks of cardboard sheets, are subjected in conventional roller conveyors. Thus, the xe2x80x9celephant footxe2x80x9d problem is reduced in severity. Because the inverted roller belt loops need not be powered, only one drive systemxe2x80x94that for driving the transport beltxe2x80x94is needed.